This invention relates to a method of and a device for controlling a stepping motor and is particularly suited to the watchmaking field.
In electronic analogue-display watches having a stepping motor to drive the display components, most of the energy supplied by the electric source, usually a battery, is used up by the motor. It is therefore important to reduce as much as possible the consumption of the motor in order either to increase the lifetime of the battery or, for a given lifetime, to diminish its volume, the space available inside a timepiece being very limited.
In most watches currently on the market, the duration of the drive pulses that are fed at regular intervals to the motor is fixed. This duration, usually 7.8 ms, is so chosen that the motor will work properly even in the worst conditions, i.e. with a low battery voltage, while driving the calendar mechanism or when the watch is subjected to shocks, external magnetic fields, etc. As these adverse conditions occur only rarely, the motor is oversupplied most of the time.
A known method of reducing the energy consumption of a motor consists in applying to it normal drive pulses of reduced duration, e.g. 3.9 ms, but long enough to ensure proper operation under optimal conditions, and in providing a device which, after each pulse, detects whether the motor has rotated or not. When no rotation occurs, the detection device causes a correcting pulse of long duration to be issued thus enabling the motor to effect the missed step. Although this system is a definite improvement over the case where the motor only receives pulses of long duration, it is not satisfactory since, whenever the motor fails to rotate in response to a normal pulse, the energy of this pulse is completely lost and the duration of the corrective pulse is usually far greater than is needed for the motor to effect its step.
Other systems use means able to detect changes in the motor load and to switch the duration or the amplitude of the drive pulses to a greater value whenever a load increase is detected. Such systems, as with the previous one, are in fact only safety devices that simply enable the motor to be issued with increased but often excessive energy when necessary.
The energy consumption of the motor can only in fact be substantially reduced by providing more sophisticated control devices that enable the energy of the drive pulses to be adapted to the momentary load on the motor and to the supply voltage.
One proposed solution has been to provide a pulse generating circuit capable of producing pulses of different durations, along with a device able to detect, as described above, the rotation of the motor or the absence of such rotation, and to reduce progressively the duration of the pulses issued to the motor until a missed or non-effected step is detected. A correction pulse of maximum duration is then issued to the motor and the energy of the normal drive pulses is adjusted to the next higher value. If the following step fails, a further incrementation is performed. Otherwise the value is maintained for a while. If the motor rotates normally during this time, the duration of the pulses is reduced again. With such a solution, a permanent and rapid adjustment of the drive pulses to the load of the motor is not possible: this adjustment in fact only proceeds from an average. Besides, as with the first system described above, the issuance of correction pulses when the motor fails to rotate involves greater energy consumption than necessary.
Some systems do enable the energy of the drive pulses to be permanently adjusted in relation to the motor load and to the battery voltage. These systems include means able to measure, while the drive pulse is applied, a parameter representative of the position or speed of the rotor, and to interrupt the pulse at an instant that is set in dependence on the time taken by the measured parameter to reach a predetermined reference level corresponding to the instant when the rotor has effected its step or has at least rotated by an angle or reached a speed sufficient to complete the step. Such a system is indeed more efficient. In practice, however, the dispersion and the variations of the characteristics of the motor and of certain components of its control circuit must be taken into account in setting the reference level. The chosen value therefore does not correspond to minimum consumption.